A method is known from U.S. Pat. No. 6,064,705 disclosing an encoding system using transmission packets including a start of frame followed by the coded data value followed by the end of frame where the start of packet encoding and the end of packet encoding is different from the data value encoding.
Optical networks face increasing bandwidth demands and diminishing fiber availability. Based on the emergence of the optical layer in transport network optical networks provide higher capacity and reduced cost. As with any new technology, many challenges arise. Bit synchronization and power adaptation at when receiving packetized data require a bit synchronization sequence. This necessity of a bit synchronization sequence results in a limitation in throughput and/or in flexibility when transmitting short packets like Internet packets.
For transmitting serialized packetized data an encoding is necessary. This encoding should be suited to encode data compact as well as suited for the data transport layer down through the physical layer, i.e. the optical medium and the used components.
Such a method of transmitting serialized packetized data is known from Mouly and Pautet, “The GSM System for Mobile Communications”, 1992, CELL & SYS., pages 231-248, where a normal burst is described containing two packets surrounding a training (or synchronization) sequence. The transmitting of NR7 bit train by converting it to synchronizing data is known from PATENT ABSTRACTS OF JAPAN Vol. 015, No. 114 (E-1047) and JP 03 006142 A.
Non-return to zero encoding (NRZ) is commonly used in slow speed communications interfaces for both synchronous and asynchronous transmission. Using NRZ, a logic 1 bit is sent as a high value and a logic 0 bit is sent as a low value, illustrated in FIG. 1.
A problem arises when using NRZ to encode a synchronous link which may have long runs of consecutive bits with the same value. In Ethernet for example, there is no control over the number of 1's or 0's which may sent consecutively. There could potentially be thousands of 1's or 0's in sequence. If the encoded data contains long ‘runs’ of logic 1's or 0's, this does not result in any bit transitions. The lack of transitions prevents the receiver from reliably regenerating the clock making it impossible to detect the boundaries of the received bits at the receiver. This is the reason why Manchester coding is used e.g. in Ethernet LANs.
Manchester encoding is a synchronous clock encoding technique often used in the physical layer to encode the clock and data of a synchronous bit stream. In this technique, the actual binary data to be transmitted are not sent as a sequence of logic 1's and 0's. Instead, the bits are translated into a slightly different format that has a number of advantages over using straight binary encoding. Manchester encoding follows the rule that an original logic 0 is encoded by a 0 to 1 change (upward transition at bit center), and a logic 1 is encoded by a 1 to 0 change (downward transition at bit center).
The diagram in the background teaching FIG. 1 shows a time diagram where the bit sequence 1,1,0,1,0,0,1,1 is in the upper part A Manchester encoded and NRZ encoded in the lower part B.
Manchester encoding may be alternatively viewed as a phase encoding where each bit is encoded by a positive 90 degree phase transition, or a negative 90 degree phase transition. The Manchester code is therefore also known as a Bi-phase Code.
A Manchester encoded signal contains frequent level transitions which allow the receiver to extract the clock signal using e.g. a digital phase locked loop (DPLL) and correctly decode the value and timing of each bit. To allow reliable operation using a DPLL, the transmitted bit stream must contain a high density of bit transitions. Manchester encoding ensures this, allowing the receiving DPLL to correctly extract the clock signal.
The penalty for introducing frequent transitions, is that the Manchester coded signal consumes more bandwidth than the NRZ encoded signal. For a 10 Mbps LAN, the signal spectrum lies between the 5 and 10 MHz.
Manchester encoding is used as the physical layer of an Ethernet LAN, where the additional bandwidth is not a significant issue. For example the pattern of bits 0, 1, 1, 1, 1, 0, 0, 1 encodes to 01, 10, 10, 10, 10, 01, 01, 10. Another more curious example is the pattern 1, 0, 1, 0, 1 which encodes to 10, 01, 10, 01, 10 which could also be viewed as “1 00 11 00 11 0”. Thus for a 10 Mbps Ethernet LAN, the preamble sequence encodes to a 5 MHz square wave i.e., one half cycle in each 0.1 microsecond bit period.
Line coding is a process of modifying a source signal to facilitate proper signal reception in the presence of transmission impairments. In optical systems employing optical intensity modulation, the required features of line codes are bit sequence independence, small low frequency content, transmission of adequate timing information, high efficiency, low error multiplication, and low systematic jitter etc. Since currently all practical systems employ optical intensity modulation, the line code is essentially uni-polar. In fiber-optic transmission, binary line codes are preferred to multilevel codes due to the inherent non-linearity of the optical media.
A typical optical transmitter consists of a digital device providing the data or payload. The data is encoded and serialized for driving a laser or modulator to get a modulated laser beam, i.e. serialization and electrical-to-optical conversion.
Emphasis is directed to the fact, that the serialized data has an upper frequency bound, which might be on the limit of the underlying technology, and that an extension of this limit would cause an intolerable increase of cost.
The receiver reverses this operation, i.e. optical-to-electrical conversion and de-serialization, with similar limitations, i.e. the speed limit of the participating components.
The invention acts on the preamble of the independent method claim for transmitting packets having a synchronization part and a payload part, sending a synchronization part, detecting the synchronization part and synchronizing and adapting the receiver for receiving the payload part.
In burst mode, i.e. when multiple packets with random length and of random arrival time are transmitted, the receiver is faced with two adaptation requirements:    (a) a fast adaptation to different power levels from packet to packet and    (b) a fast adaptation to different bit clock phases from packet to packet.
Both requirements result in a transient with for a certain transient time, for both amplitude as well as for bit phase. At the beginning of each packet, during the transient, a receiver is not able to interpret received data correct. To cope with this fact, a bit synchronization sequence in front of the packet is needed to absorb the transient before arrival of any relevant data bit.
The transients in the receiver have lower bounds. It has to be large enough not causing transients by regular data patterns. There is a relationship between data rate, transient times, and the length of bit synchronization sequence.
The bit synchronization sequence is lost in terms of throughput, and when it is of fix length, the smaller the packets the larger the percentage of loss.
To overcome this dilemma two solutions are known:                (1) Do a rigid line coding of the whole data packet, which is alternating (DC free) even for a small number of bits, e.g. Manchester coding. This would allow to reduce the transient time to a minimum, for the price of loosing half of the data rate.        (2) A variable (or switchable) transient time. Short transient during bit synchronization sequence, but long hold time during the data packet. The change of transient time needs to be completed within the timeframe of the bit synchronization sequence resulting in a new timing constraint: The change or switch of transient time is initiated by a start of packet detector. Hence the bit synchronization sequence cannot be shorter than the delay in the start of packet detection path.        
There is already a solution available concerning the clock recovery and for adapting the amplitude level, e.g. European patent application EP 1 221 781.
The change of the transient time needs to be completed within the timeframe of the bit synchronization sequence, producing a new timing limitation. The change or switch of the transient time is initiated by a detector recognizing the start of a packet. That is the reason why the bit synchronization sequence cannot be shorter than the delay in the start of packet detection path.
This invention targets the problem of improving the (asynchronous and synchronous) coding tradeoff for burst transmission and burst switching. Bit synchronization and power adaptation at packet start require a bit synchronization sequence. The necessity of a bit synchronization sequence results in a limitation in throughput and/or in flexibility when transmitting short packets.
Using Manchester coding or other strong line coding, having the disadvantage of loosing about half of the data rate, if the bandwidth is fix. The observation is that strong line coding seems to be well suited for small packets, where the switching is the major target.
Switchable timing of the receiver, having the disadvantage of a control signal for the time constant switch causing a timing limitation for the bit synchronization sequence. This is good for large packets, where the throughput is major target.
These problems and restrictions are overcome by a method for transmitting packets comprising a synchronization part and a payload part, comprising the steps of sending a synchronization part using a first encoding, encoding and sending the payload part using a second encoding, on the sender side and detecting the synchronization part in the first coding and synchronizing and adapting the receiver and decoder, receiving and decoding the payload part in the second encoding on the receiver side, wherein the transmission format comprises a shortened synchronization part and the payload part is split into a first data sequence, encoded in the first encoding, followed by the second data sequence, encoded in the second encoding, comprising further interleaved steps of encoding and sending the first data sequence in the first encoding, encoding and sending the second data sequence in the second encoding, on the sender side and receiving and decoding the first data sequence in the first encoding, detecting the end of the first data sequence and adapting the decoder, receiving and decoding the second data sequence in the second encoding on the receiver side. The first data sequence might be encoded in a line coding enabling a synchronization in the receiver. And this first data sequence might be Manchester encoded. The second data sequence might be non-return-to-zero encoded. The adaptation and the decoding might be depended on the received signal pattern or might be time depended. The second encoding might comprises further encodings.
These problems and restrictions are overcome by a sender for transmitting packets comprising a synchronization part and a payload part, the sender comprising a serialization unit for serialization data and an encoding unit for encoding said serialized data, wherein said serialization unit comprising a synchronization unit for generating the synchronization part and an encoding unit for generating a first data sequence, encoded in a first coding, comprising a reserve part, followed by the second data sequence, encoded in a second coding.
Correspondingly these problems and restrictions are overcome by a Receiver for transmitting packets comprising a synchronization part (sync′) and a payload part, said receiver comprising a detector unit for detecting a synchronization part, a control unit for adapting the receiver characteristics, wherein according to said detected synchronization part, a receiver unit for decoding a first data sequence, encoded in a first encoding, said detector unit detecting the end of said first data sequence, said control unit adapting the receiver's decoding, and said receiver unit for decoding a second data sequence, encoded in a second encoding.
These problems and restrictions are furthermore overcome by an Optical Networking Element for transmitting packets comprising a synchronization part and a payload part, said optical networking element comprising a sender comprising a serialization unit for serialization data, an encoding unit for encoding said serialized data, wherein the serialization unit comprising a synchronization unit for generating a synchronization part and an encoding unit for generating a first data sequence, encoded in a first coding, comprising a reserve part, followed by the second data sequence, encoded in a second coding.
And these problems and restrictions are overcome by an Optical Networking Element for transmitting packets comprising a synchronization part (sync′) and a payload part, said optical networking element comprising a receiver comprising a detector unit for detecting a synchronization part, a control unit for adapting the receiver characteristics, wherein according to said detected synchronization part, a receiver unit for decoding a first data sequence, encoded in a first encoding, said detector unit detecting the end of said first data sequence, said control unit adapting the receiver's decoding, and said receiver unit for decoding a second data sequence, encoded in a second encoding.
These problems and restrictions are overcome by a Serialized Packet Format for transmitting packets comprising a synchronization part and a payload part, wherein a first data sequence in a first coding and, separated, a second data sequence encoded in a second coding. The first data sequence might be Manchester encoded and the second data sequence might be non-return-to-zero encoded.